Many applications, such as automotive and aircraft fabrication, require precisely positioned holes to be drilled in a workpiece. The holes must be accurately aligned with only relatively small positioning errors tolerable. Accordingly, a drill plate is typically aligned and mounted in a fixed relation to the workpiece prior to drilling holes in the workpiece. The drill plate has one or more accurately positioned holes extending therethrough such that by properly aligning the drill plate with respect to the workpiece, a hole in the drill plate precisely overlies the desired location for a corresponding hole to be drilled in the workpiece.
In many instances, it is desirable to attach the drill motor to the drill plate such that the operator does not have to offset the full weight of the drill motor and the thrust generated during the drilling operation. Attachment of the drill motor to the drill plate further ensures proper alignment of the cutting tool with the desired hole location in the workpiece. In many instances, the accurately positioned holes must be drilled in relatively hard materials, such as titanium. To drill holes in such materials, it is desirable to use power feed or rack feed drill motors which produce relatively great amounts of axial thrust. Typically, these motors are internally designed so that the drill spindle and cutting tool are actuated with pneumatic and/or hydraulic power or with the assistance of a rack and pinion drive. Since these motors are generally heavy and produce a relatively great amount of thrust, however, the drill motor is generally firmly clamped to a drill plate and the drill plate is, in turn, firmly clamped to the workpiece. Accordingly, the affixation of a drill motor to a drill plate eliminates the need for the drill motor operator to apply axial pressure during the drilling operation.
Power feed drill motors are commonly attached to a drill plate by means of lock screws, lock buttons or specially designed liner bushings. For example, a pair of lock screws may be positioned on opposite sides of an aperture defined in the drill plate. Lock screws typically have an unthreaded portion underlying the slotted head for receiving a corresponding flange of a locking drill bushing that is mounted to the nose housing of a drill motor. Accordingly, the locking drill bushing of the drill motor is inserted into an aperture defined in a drill plate and rotated so that the flange of the locking drill bushing is engaged and retained beneath the slotted head of the lock screws. The drill motor is thus affixed to the front surface of the drill plate.
The use of lock screws, lock buttons or specially designed liner bushings in the drill plate typically increases both the cost of drill plates and the labor cost associated with using these drill plates. In particular, the expense of the lock screws, lock buttons and liner bushings as well as the labor costs for installation and maintenance increase the cost of the drill plates. In addition, lock screws, lock buttons, and liner bushings increase the size and weight of drill plates and preclude the close spacing or overlapping of apertures in the drill plate. Thus, additional or multiple drill plates may be required for a single part.
In order to securely clamp a drill motor to a drill plate without the use of lock screws, specially adapted sleeves and bushings have been designed for incorporation into the drill plate. Examples of such sleeves and bushings incorporated in drill plates include those described in U.S. Pat. No. 5,161,923 (the '923 patent) to Helmut Reccius on Nov. 10, 1992 and U.S. Pat. No. 4,740,117 (the '117 patent) to Marie-Jose Schaff Deleury et al. on Apr. 26, 1988. The drill plate described in the '923 patent includes a cylindrical protrusion extending outwardly from the front face of the drill plate and a recessed periphery about the aperture on the rear face of the drill plate to align and support the drill motor. The machine tool of the '923 patent includes a nosepiece having a number of longitudinal tongues. The tongues are adapted to expand within the aperture of the drill plate and be received within the recessed periphery of the aperture.
Further, the '117 patent discloses drill bushing inserts or sleeves within apertures defined in the drill plate. The inserts have a cylindrical portion extending outwardly from the front face of the drill plate about each aperture. The cylindrical portion has a flanged end for mating with the correspondingly designed drill motor. In particular, the drill motor has a plurality of protruding fingers having a flanged end portion adapted to mate with the flanged end of the cylindrical portion.
In addition, U.S. Pat. No. 2,839,953 (the '953 patent) to Randolph T. Hanger on Jun. 24, 1958 also incorporates a sleeve within the aperture defined in the drill plate. The drill bushing of the '953 patent is manually expanded within the tapered sleeve to attach the drill motor to the drill plate.
The fabrication of specially designed drill plates, however, increases the cost of the drill plates and the time required for their manufacture. The use of sleeves or bushings within the apertures of a drill plate also prevents the incorporation of overlapping apertures in the drill plate. In addition, such specially designed drill plates may generally only be used with drill motors having bushings designed to mate with the corresponding sleeves or bushings of the drill plate.
Drill bushings have also been specially designed for attachment to drill motors in order to clamp the drill motor to a drill plate. For example, U.S. Pat. No. 2,935,905 (the '905 patent) to J. C. Winslow on May 10, 1960 discloses a drill bushing adapted for extension through an aperture in a drill plate and for attachment thereto. The drill bushing of the '905 patent incorporates a pneumatically-actuated ram assembly for extending the drill bushing through tile aperture in the drill plate.
The ram assembly of the '905 patent, however, is offset from the cutting tool and is an integral component of the drill motor itself. This configuration generally requires dedicated motors for specific drill applications which are generally relatively expensive. The offset ram assembly can also tend to bind, when activated, which may compromise proper alignment of the cutting tool and the production of quality holes in the workpiece. The offset configuration also requires additional space which may limit lateral movement of the drill motor and prevent access to all apertures in the drill plate.
Thus, while it would be desirable to securely clamp a drill motor to a drill plate in order to drill precisely aligned holes, such clamping of a drill motor to a drill plate still suffers from a number of deficiencies. These deficiencies include, for example, the need for specially designed bushings or sleeves in the drill plate, the use of expensive dedicated drill motors with offset integral ram assemblies, and the inability to reach areas of limited lateral access. These deficiencies are particularly apparent in instances which require relatively large clamping forces including instances in which a drill motor must produce relatively large amounts of thrust such as, for example, drilling holes in relatively hard materials with power feed drill motors.